I have not yet seen this upgrade being used when I've played, nor have I obtained this upgrade yet. But I saw Wolfy Playz and German Shu use it, also checked to find how this upgrade actually works! I love his shield, but don't like damage over speed as much. I prefer speed over damage, when possible. If you want to know exactly how the upgrade works, from my research, here's how it works, with Shogun-Guard equipped on Kernel Corn:
- Corn does 11 damage with his primary weapon. This can be increased with Butter Beacon.
 - Corn shoots at about 55-60% speed, compared to if not using this upgrade.
 - Corn's shield has 75 health.
 - When not using the shield, it takes 2 seconds for it to start regenerating.
 - If the shield is destroyed, it takes 5 seconds for it to start regenerating instead.
 - The shield regenerates 10 health per second.
 - You can shoot while shielding as corn.
 
Overall, I haven't tried this upgrade, but from what I've seen, I really like it and can't wait to use it! 🙂